Apparatus and methods for measuring ion mass as a function of mobility

ABSTRACT

Ion mass is determined directly as a function of the drift time in a chamber containing gas at atmospheric pressure. The drift time is a function of approximately the cube root of the ion mass for ion mass greater than the mass of the drift gas and a function of approximately the square root of the ion mass for ion mass less than the mass of the drift gas. A measure of the ion mass may be obtained by recording the ion-current output of the drift tube with the time base adjusted to an exponential function of the drift time. Corrections may be made automatically for variations in temperature, pressure and drift field.

United States Patent 11 1 Wernlund et al.

1111 3,812,355 1451 May 21, 1974 [54] APPARATUS AND METHODS FOR2.780.728 2/1971 Langmuir 250/419 TF MEASURING O MASS AS A FUNCTION2,772.364 11/1956 Washburn 250/419 TF F MOBILITY 3,668,384 6/1972Mooreman et al 250/419 TF l 1 F. l [75] memo 335? I zgg s gxg e 2H2Przmary Exammer.lames W. Lawrence D t k th P11 B l Assistant Examiner-B.C. Anderson gj g i x iz gi Attorney, Agent, or FirmRaphael Semmes all ofFla.

[73] Assignee: Franklin Gno Corporation, West [57] ABSTRACT al Beach,Fla. [on mass is determined directly as a function of the [22] Filed: 91971 drift time in a chamber containing gas at atmospheric pressure. Thedrift time is a function of approximately PP NOJ 206,353 the cube rootof the ion mass for ion mass greater than the mass of the drift gas anda function of approxi- 52 US. Cl 250/283 match the Square root 0f theion mass for ion mass iii Int. Cl. IIOIIFZE/IS I l than the mass of thedrift A measure of the 58 Field of Search. 250/410 TF, 41.9 SE, 41.9 G,mass may Q h hy recordlhg the 250/419 current output of the dr1ft tubew1th the time base adjusted to an exponential function of the drifttime. [56] References Cited Corrections may be made automatically forvariations UNITED STATES PATENTS in temperature, pressure and driftfield. 1626,180 12/1971 Caroll et a1. 250/419 TF 23 Claims, 6 DrawingFigures T (49 P 760 so ABsOuJTE PRESSURE V DIVIDER TRANSDUCER ABSOLUTETEMPERATURE P TRANSDUCER 27211 V HIGH VOLTAGE POWER SLPPLY R2 oc )B TMULTIPLIER MULTIPLIER [L 273 P T We I m -r( 1-d E T 760 v3- Ko- PF 3010) 32 (54 1/ =k1- PLASMA PLASMA DIODE CHROMATOGRAPH CHROMATOGRAPHWLTIPLlER FUNCTION XAXIS CHAMBER CONTROLLER k eENERATom I 3565x513" LRAM: VOLTAGE v M 52 IVS DRIFT MASS ELgCTROMETER Y Axis 2a I IFATENTEUEAY 2: 19m

I I 8 IO TIME IN MILLISECONDS I .LNHHHHO NOI I lNaaano NOI DRIFT MASSF-ATENTED HAY 2 1 i974 SHEEI t 0? 4 LOOO IOO

REDUCED MOBILITY APPARATUS AND METHODS FOR MEASURING ION MASS AS AFUNCTION OF MOBILITY BACKGROUND OF THE INVENTION This invention relatesto Plasma Chromatography and is more particularly concerned with themeasurement of ion mass directly from a Plasma Chromatograph chamberoperating at atmospheric pressure.

The basic concepts of Plasma Chromatography are now well-known. See, forexample, The Plasma Chromatograph, Research/Development, March, 1970. Inthe Plasma Charomatograph, which preferably operates at atmopshericpressure, product ions are produced by ion-molecule reactions, and theions are caused to drift in an electric field toward a detector,becoming segregated in accordance with their mobility in the driftfield. The output of the Plasma Chromatograph may be a recording inwhich ion species are represented by the peaks of a curve of ion currentversus drift time. In order to measure the mass of the ion species, ithas been necessary to inject ions from the Plasma Chromatograph chamberinto a conventional mass spectrometer (lacking the usual ion source).

BRIEF DESCRIPTION OF THE INVENTION It is a principal object of thepresent invention to provide a measure of ion mass directly from thePlasma chromatograph.

Another object of the invention is to provide a method and apparatus formeasuring the mass of trace molecules at atmospheric pressure.

Still another object of the invention is to provide apparatus and methodfor measuring the mass of molecules over a very wide range, and up tothe order of 5,000 to 10,000 atomic mass units.

A further object of the invention is to provide simple instrumentationfor measuring the mass of molecules on a linear or logarithmic scale.

A still further object of the invention is to provide instrumentation ofthe foregoing type in which variations due to pressure, temperature, ordrift field changes may be compensated automatically or manually.

Briefly stated, in a preferred embodiment of the invention tracemolecules are converted to ions by ionmolecule reactions and aresubjected to a drift field in a chamber containing a drift gas atatmospheric pressure. The ions are segregated in accordance with theirmobility in the drift field and are collected to produce an ion current,pulses of which correspond to ion species of different mobility. Byrecording these pulses with respect to a time base which variesexponentially as a function of drift time, a direct measure of the massof the molecules (ions) is obtained.

BREIF DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed in conjunction with the accompanying drawings, whichillustrate preferred and exemplary embodiments, and wherein:

FIG. 1 is a block diagram of a system in accordance FIG. 3 is a waveformdiagram illustrating the output of the Plasma Chromatograph as afunction of drift time;

FIG. 4 is a graphical diagram illustrating the relationship ofdrift-mass and reduced mobility in accordance with the invention;

FIG. 5 is a graphical diagram illustrating the relationship ofdrift-mass and time of drift in accordance with the invention; and

FIG. 6 is a waveform diagram illustrating the output of an instrument ofthe invention from which drift-mass can be read directly.

A basic Plasma Chromatograph chamber I0 is shown in FIG. 2 and comprisesan envelope I2 containing a pair of spaced electrodes Hand 16 adjacentto opposite ends of the envelope, the electrodes being separated byseveral centimeters, for example. An inlet 18 is provided adjacent toelectrode 14 for the admission of a sample, and an outlet 20 is providedalso. An ionizer 21 is provided adjacent to electrode 14 and maycomprise a tritium film supported on the electrode, for example. Asample, which may comprise a host gas,

such as air at atmospheric pressure, containing trace molecules, such asDMSO, is admitted by the inlet 18, passes the ionizer 21, and leaves theenvelope by way of the outlet'20. Reactant ions of a reactant gas, whichmay be part of the sample, are produced by the ionizer, and product ionsof the trace molecules are formed as the result of ion-moleculereactions involving the reactant ions. The length of the mean free pathof the ions is very much less than the dimensions of the reactionregion.

An electric drift field is established between electrodes l4 and 16 by ahigh voltage power supply. The polarity of the field is selected tocause the ions to drift from the reaction region adjacent to electrode14 toward the collection region at electrode 16. A nonreactive (inert)gas, such as nitrogen, is admitted to the chamber by an inlet 22,leaving the chamber by way of outlet 20. The drift gas fills the driftregion between a pair of spaced shutter grids 24 and 26, the first ofwhich is in the vicinity of the electrode 14 and the second of which isadjacent to the collector electrode 16. Each grid comprises a pair ofcoplanar, interdigitated, parallel-wire grid sections, whereby alternatewires of the grid may normally be held at equal and opposite potentialswith respect to a grid average potential, which may be applied to thegrids from taps of a voltage divider across the high voltage powersupply. Uniformity of field between the electrodes 14 and 16 may bemaintained by a series of guard rings (not shown) also connected to tapsof the voltage divider.

A mixed ion population, represented by the letters A, B, and C in FIG.2, is presented to the first shutter grid 24 from the reaction regionbetween electrode 14 and shutter grid 24. At a predetermined time theshutter grid is opened, by driving each of the grid sections to the gridaverage potential, and remains open long enough to pass a group of ionsto the drift region between the grids 24 and 26. The various ionspecies, A, B, and C, become segregated in the drift region inaccordance with their mobility, each species reaching substantiallyconstant statistical drift velocity characteristic of the ion species.The separate species may then be passed to the collector 16 by openingthe second shutter grid 26 at an appropriately delayed time with respectto the opening of the first shutter grid 24. The collector may beconnected to an electrometer, for example, which integrates the ioncurrent over successive cycles. By scanning the time of opening of grid26 relative to 'grid 24, substantially the entire ion population withinthe drift region may produce output pulses as a function of drift timeas shown in FIG. 3.

The Plasma Chromatograph has very high sensitivity for the detection oftrace molecules capable of engaging in ion-molecule reactions, but inthe past it. has been necessary to employ a conventional massspectrometer in tandem with the Plasma Chromatograph chamber in order todetermine the mass of the molecules (ions). In accordance with thepresent invention, however, it has been discovered that a measure of themass can be taken directly from the Plasma Chromatograph output, therebymaking it possible to perform mass measurements at atmospheric pressureand avoiding the need for the complexity and high vacuum of the massspectrometer. The term drift-mass will be utilized herein to connote themeasured ion mass, to account for the fact that in some instances themeasured mass may be affected by the'molecule configuration. The termreduced mobility connotes the measured mobility normalized to standardtemperature and pressure conditions.

As shown in FIG. 4, it has been discovered that there is an exponentialrelationship between the drift mass (expressed in atomicmass units) andthe reduced mobility When the ion mass M,- is less than the mass of thedrift gas M the mobility approaches a Langevin polarization dependence,K -(M,) where M, is the reduced mass defined by HM, l/M, I/M For M,-greater than M,,, the reduced mobility K approaches an inverse cubicdependence, K -M,-" This is illustrated by the curve of FIG. 4 (wheredrift mass is on a logarithmic scale), the portion to the right of thevertical line being in accordance with the square root relationship andthe remainder of the curve, to the left of the vertical line, being inaccordance with the cube root relationship. The region where M,- M is atransition region. Where the drift air or nitrogen at atmosphericpressure, it has been discovered that the square root relationship holdsup to M,--50 to I00 AMU, and that above this, the cube root relationshipexists. Since the time of drift is inversely proportional to the reducedmobility, the same relationship may be shown in terms of drift massversus time of drift as in FIG. 5 (drift-mass being shown on a linearscale).

FIG. 1 illustrates a system of the invention for obtaining a directreadout of drift mass. In effect, this system converts the drift timeaxis of FIG. 2 to a drift-mass axis. The Plasma Chromatograph chamber(previously described) has its output connected to the electrometer 28,which integrates the ion current as described previously. The output ofthe electrometer is connected to the Y-axis input of an X-Y recorder 30which is employed to produce the direct readout of ion current versusdrift mass as shown in FIG. 6. In order that the Y-axis ion currentpulses from the electrometer 28 may be positioned along the X-axis so asto produce a direct readout of drift mass, d a time base v must begenerated in accordance with the equation:

trary constant). If the mass of the drift gas is low enough, themolecules of interest may have a mass greater than that of the driftalone may suffice.

The reduced mobility K, is related to the electric field E (volts percentimeter), drift length L (centimeters), absolute temperature T(degress Kelvin), absolute pressure P (Torr) and drift time 1 (seconds)by the relationship l/Ko E/L T/273 760/P x 1. Thus, to obtain thevoltage v;,, the factors E, L, T, P, and t must be considered.

As shown in FIG. 1, a ramp voltage v, =kt is obtained from the PlasmaChromatograph controller 32. This is a ramp voltage generatorsynchronized with the pulse which opens the first shutter grid 24 of thePlasma Chromatograph chamber. Each time the shutter grid 24 is opened,the ramp voltage generator commences the generation of a ramp voltage vthe amplitude of which varies as a linear function of time t. The rampvoltage v is applied to one input of a multiplier 34, the other input ofwhich is a voltage equal to E/L T/273 760/P. This voltage is'obtained asfollows: A voltage E/L is derived from a voltage divider comprisingreisitors 36 and 38 connected in series across the high voltage powersupply 40 from which the drift voltage V is obtained. If the appliedvoltage to the chamber is V (volts) and the overall length of thechamber between the electrodes 14 and 16 is L, (centimeters) then E/LV/L L, and resistors 36 and 38 are adjusted in value to produce thevoltage E/L at the voltage divider tap. This is applied as one input toa multiplier 41. The other input is a voltage equal to T/273. Thisvoltage is obtained by applying the voltage output of an absolutetemperature transducer 42 to a voltage divider comprising resistors 44and 46 in series, the values of which are adjusted to produce thevoltage T/273 at the divider tap. The absolute temperature transducermeasures the absolute temperature of the gas in the Plasma Chromatographchamber. Any conventional temperature transducer which produces anoutput voltage which varies linearly with absolute temperature may beemployed. The product output of multiplier 41 is a voltage E/L T/2'73,and this is applied as one input to a multiplier 48, the other input ofwhich is a voltage equal to 760/ P. This voltage is obtained by applyinga voltage P, the output of an absolute pressure transducer 49, to adivider 50, the other input of which is a voltage equal to 760. Thepressure transducer measures the pressure of the gas in the PlasmaChromatograph chamber and may be any conventional absolute pressuretransducer producing an output voltage which varies linearly withpressure.

The product output of multiplier 48 is the voltage E/L X T/273 X 760/P,which is applied to multiplier 34.

The product output of multiplier 34 is .a voltage v kt X E/L X 77273 X760/P. t.

This is applied to a diode function generator 52, which generates thevoltage v referred to previously.

The multipliers and dividers referred to above may be operationalamplifiers connected to muliply or to divide. The function generator 52may be an operational amplifier diode function generator, the successivesections of which may be adjusted to approximate many arbitraryfunctions. The multiplier or the divider may be a Teledyne PhilbrickNexus Model 4450 or gas and the cube function I Model 4452, or BurrBrown Model 4029/25 or Model 4030/25. The diode function generator maybe a Burr Brown Model 4062/45 or a Teledyne Philbrick Nexus Model SPFX.I

The diode function generator may be set empirically so as to produce anX-axis time base which varies in the manner set forth above. Thus, testruns may be made with known ion species and a plot of drift mass (known)versus time of drift (measured) obtained as in FIG. 5. The functiongenerator 52 may then be adjusted to modify the time base so that theion current pulses (Y-axis) are produced at times linearly (orlogarithmically, if desired) proportional to drift mass, as indicated inFIG. 6.

If desired, one or more of the functions providing automatic correctionof electric field, temperature, or pressure may be replaced by acalibrated potentiometer, which may be manually set to the proper valuecorresponding to the condition in the plasma Chromatograph chamber, bybreaking the circuit at point A-, B, or C and substituting theappropriate manually set voltage. The unused portions of the circuitwould then be replaced by the appropriate voltage, temperature, orpressure indicator.

From the foregoing, it is apparent that the invention permits ameasurement of drift mass by a Plasma Chromatograph directly.Empirically, it has been found that reduced mobility is characteristicof the mass of each ion species when measurements are performed at 760Torr and E/P-0.5 volts/Torr-centimeter. The relationship between ionmass and reduced mobility extends to very high ion mass values, of theorder of 10,000 AMU. The measurement of drift-mass in accordance withthe invention produces highly useful data which may be used inconjunction with other data, such as the output of a gas chromatographor the output of another Plasma Chromatograph drift-mass instrumentusing a different drift gas. By employment of such a family ofinstrumentation, chemical identification is possible.

In some instances two molecules of identical mass but of differentatomic arrangement may result in different mobility measurements. Wherethe mass of the ion is the same as or double the drift gas mass,spurious ion-molecule resonance effects can affect the results.Therefore, the ion mass may be considered as lying in a band centered onthe drift mass-mobility calibration curve rather than directly on thecurve.

Resolution may sometimes be improved by employing a heavier drift gasthen a drift gas such as nitrogen, for example. With nitrogen as thedrift gas (molecular weight 28), an ion with the molecular weight of 10times this (280) loses drift-mass resolution from a mo] While preferredembodiments of the invention have been shown and described, it will beapparent to those skilled in the art that changes can be made in theseemspirit of the invention, the scope of which is defined in the appendedclaims.

The invention claimed is:

1. In a method wherein ions are caused to drift through a drift spacecontaining a drift gas of known mass and at a pressure sufficient toensure that the length of the mean free path of said ions in said spaceis substantially less than the dimensions of said space, the improvementfor producing a measure of ion mass which comprises producing an outputwhich varies approximately as a cube function of the drift time for ionmasses greater than the atomic mass of the drift gas.

2. A method in accordance with claim 1, wherein said output is producedapproximately as a square function of the drift time for ion masses lessthan the mass of the drift gas.

3. A method in accordance with claim 1, wherein the ions are produced byion-molecule reactions in a reaction space maintained at a pressure suchthat the length of the mean free path of the ions is substantially lessthan the dimensions of the reaction space.

4. A method in accordance with claim 1, wherein the pressure is suchthat the ions reach substantially constant statistical drift velocity inthe drift space.

5. A method in accordance with claim 1, wherein the ions are caused todrift in said space by the application of an electric drift fieldthereto.

6. A method in accordance with claim 5, wherein the producing of saidoutput comprises producing an ion current at a time delayed with respectto the commencement of ion drift.

7. A method in accordance with claim. 6, wherein the producing of saidoutput comprises making a recording of said ion current along atime basewhich varies as an exponential function of the drift time.

8. A method in accordance with claim 7, wherein the time base varies asa function of the temperature and pressure ofthe gas in said space andthe potential across. said space.

9. A method in accordance with claim 6, wherein said ions are segregatedin accordance with their mobility in the drift spaceand the ion currentis produced in response to the segregated ions. 7

10. A method in accordance with claim 8, wherein the segregating of theions'comprises gating a group of ions into the drift space and latergating a portion of the group to an ion collector.

11. In apparatus including a drift chamber, means including a pair ofspaced electrodes for producing a drift field in said chamber, meansproviding a drift in said chamber at a pressure such that the length ofthe mean free path of ions in said chamber is substantially less thanthe dimensions'of the drift space between said electrodes, means forproviding ions adjacent to one of said electrodes, means for producingan ion current in response to ions adjacent to the other electrode, theimprovement which comprises means for producing an output measure of ionmass in response to said ion current as an exponential function of thedrift time.

12. Apparatus in accordance with claim 11, said function being initiallyapproximately a square function of the drift time and then beingapproximately a cube function of the drift time.

13. Apparatus in accordance with claim 11, said function beingapproximately a cube function of the bodiments without departing fromthe principles and drift time.

14. Apparatus in accordance with claim 1 1, said output producing meanscomprising a recorder having a time base which varies in accordance withsaid function.

15. Apparatus in accordance with claim 11, wherein said functionincludes factors corresponding to the temperature and pressure of saidgas and the strength of said field.

16. Apparatus in accordance with claim 11, said output producing meanscomprising means for generating a potential.

17. Apparatus in accordance with claim 16, wherein said potentialgenerating means comprises means for producing a ramp-voltage and anexponential function generator controlled by said ramp voltage.

18. Apparatus in accordance with claim 17, wherein said functiongenerator is a diode-type function generator.

19 Apparatus in accordance with claim 17, wherein said drift chamber hasmeans for initiating a drift period synchronously with the commencementof said ramp voltage. a

, 20. Apparatus in accordance with claim 17, further comprising meansfor producing a voltage proportional to the drift field, means forproducing a voltage proportional to the absolute temperature of saidgas, means for producing a voltage inversely proportional to theabsolute pressure of said gas, and means for obtaining the product ofsaid voltages and controlling the function generator in responsethereto.

21. Apparatus in accordance with claim 17, said output producing meanscomprisinga recorder having orthogonal coordinate axes, one of which isa time base axis, means for controlling the time base in accordance withthe output of said function generator, and means responsive to the ioncurrent for controlling recording along the remaining axis.

22. Apparatus in accordance with claim 11, wherein said ion providingmeans comprises means including an ion source adjacent to said oneelectrode for providing product ions by ion-molecule reactions.

23. Apparatus in accordance with claim 11, wherein said chamber has apair of ion gates spaced apart between said electrodes and means foropening and gates sequentially, whereby ions produced adjacent to saidone electrode are admitted to the space between said gates and becomesegregated in accordance with their mobility and whereby a portion ofthe segregated ions is passed by the second gate to the other electrode.

1. In a method wherein ions are caused to drift through a drift spacecontaining a drift gas of known mass and at a pressure sufficient toensure that the length of the mean free path of said ions in said spaceis substantially less than the dimensions of said space, the improvementfor producing a measure of ion mass which comprises producing an outputwhich varies approximately as a cube function of the drift time for ionmasses greater than the atomic mass of the drift gas.
 2. A method inaccordance with claim 1, wherein said output is produced approximatelyas a square function of the drift time for ion masses less than the massof the drift gas.
 3. A method in accordance with claim 1, wherein theions are produced by ion-molecule reactions in a reaction spacemaintained at a pressure such that the length of the mean free path ofthe ions is substantially less than the dimensions of the reactionspace.
 4. A method in accordance with claim 1, wherein the pressure issuch that the ions reach substantially constant statistical driftvelocity in the drift space.
 5. A method in accordance with claim 1,wherein the ions are caused to drift in said space by the application ofan electric drift field thereto.
 6. A method in accordance with claim 5,wherein the producing of said output comprises producing an ion currentat a time delayed with respect to the commencement of ion drift.
 7. Amethod in accordance with claim 6, wherein the producing of said outputcomprises making a recording of said ion current along a time base whichvaries as an exponential function of the drift time.
 8. A method inaccordance with claim 7, wherein the time base varies as a function ofthe temperature and pressure of the gas in said space and the potentialacross said space.
 9. A method in accordance with claim 6, wherein saidions are segregated in accordance with their mobility in the drift spaceand the ion current is produced in response to the segregated ions. 10.A method in accordance with claim 8, wherein the segregating of the ionscomprises gating a group of ions into the drift space and later gating aportion of the group to an ion collector.
 11. In apparatus including adrift chamber, means including a pair of spaced electrodes for producinga drift field in said chamber, means providing a drift in said chamberat a pressure such that the length of the mean free path of ions in saidchamber is substantially less than the dimensions of the drift spacebetween said electrodes, means for providing ions adjacent to one ofsaid electrodes, means for producing an ion current in response to ionsadjacent to the other electrode, the improvement which comprises meansfor producing an output measure of ion mass in response to said ioncurrent as an exponential function of the drift time.
 12. Apparatus inaccordance with claim 11, said function being initially approximately asquare function of the drift time and then being approximately a cubefunction of the drift time.
 13. Apparatus in accordance with claim 11,said function being approximately a cube function of the drift time. 14.Apparatus in accordance with claim 11, said output producing meanscomprising a recorder having a time base which varies in accordance withsaid function.
 15. Apparatus in accordance with claim 11, wherein saidfunction includes factors corresponding to the temperature and pressureof said gas and the strength of said field.
 16. Apparatus in accordancewith claim 11, said output producing means comprising means forgenerating a potential.
 17. Apparatus in accordance with claim 16,wherein said potential generating means comprises means for producing aramp voltage and an exponential function generator controlled by saidramp voltage.
 18. Apparatus in accordance with claim 17, wherein saidfunction generator is a diode-type function generator.
 19. Apparatus inaccordance with claim 17, wherein said drift chamber has means forinitiating a drift period synchronously with the commencement of saidramp voltage.
 20. Apparatus in accordance with claim 17, furthercomprising means for producing a voltage proportional to the driftfield, means for producing a voltage proportional to the absolutetemperature of said gas, means for producing a voltage inverselyproportional to the absolute pressure of said gas, and means forobtaining the product of said voltages and controlling the functiongenerator in response thereto.
 21. Apparatus in accordance with claim17, said output producing means comprising a recorder having orthogonalcoordinate axes, one of which is a time base axis, means for controllingthe time base in accordance with the output of said function generator,and means responsive to the ion current for controlling recording alongthe remaining axis.
 22. Apparatus in accordance with claim 11, whereinsaid ion providing means comprises means including an ion sourceadjacent to said one electrode for providing product ions byion-molecule reactions.
 23. Apparatus in accordance with claim 11,wherein said chamber has a pair Of ion gates spaced apart between saidelectrodes and means for opening and gates sequentially, whereby ionsproduced adjacent to said one electrode are admitted to the spacebetween said gates and become segregated in accordance with theirmobility and whereby a portion of the segregated ions is passed by thesecond gate to the other electrode.